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Direct observation of thermal spin depolarization in a type IB half-metal NiCo2O4 by spin-resolved photoemission

Hirokazu Fujiwara1,2,*, Tomoki Higashikawa3, Enda Xiao4, Terumasa Tadano4, Lingling Xie5, Daisuke Kan5, Yuichi Shimakawa5, Kaishu Kawaguchi6, Ryo Mori6,7 et al.

Ayumi Harasawa6, Takeshi Kondo6,8, and Takayoshi Yokoya3,9

  • *Contact author: hfujiwara@issp.u-tokyo.ac.jp
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Phys. Rev. Research 8, 043099 – Published 29 October, 2026

DOI: https://doi.org/10.1103/9hsj-vmwn

Abstract

Half-metals (HMs), characterized by 100% spin polarization at the Fermi level (EF), are ideal candidates for future spintronic memory devices. However, their spin polarization rapidly decreases with temperature, limiting their practical implementation. While this degradation has been theoretically attributed to nonquasiparticle (NQP) states—many-body electronic states arising from electron-magnon interactions that reduce spin polarization near EF—experimental verification remains limited. Here, we report the temperature evolution of the electronic states in epitaxial NiCo2O4 (NCO) thin films, a predicted type IB HM where the main spectral weight of NQP states lies below EF. Using high-resolution bulk-sensitive laser-based spin-resolved photoemission spectroscopy, we reveal that at 20 K, a metallic Fermi edge is observed in the minority-spin states but is absent in the majority-spin states, providing spectroscopic evidence for the half-metallicity of NCO. As temperature increases, majority-spin spectral weight emerges at EF, and spin polarization near EF decreases—consistent with the emergence of NQP states. The onset of this behavior occurs around 100 K. Our results suggest that NCO provides a promising platform for investigating NQP-related many-body effects in type IB HMs, guiding the design of spintronic materials with robust high-temperature spin polarization.

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Phys. Rev. Research 8, 043099– Published 29 October, 2026
  • Received 11 November 2025
  • Revised 19 April 2026
  • Accepted 2 October 2026
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

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